Hoff B W, Mardahl P J, Gilgenbach R M, Haworth M D, French D M, Lau Y Y, Franzi M
Department of Nuclear Engineering and Radiological Sciences, Plasma, Pulsed Power and Microwave Laboratory, University of Michigan, Ann Arbor, Michigan 48109, USA.
Rev Sci Instrum. 2009 Sep;80(9):094702. doi: 10.1063/1.3233917.
Experiments have been performed on the UM/L-3 (6-vane, L-band) relativistic magnetron to test a new microwave window configuration designed to limit vacuum side breakdown. In the baseline case, acrylic microwave windows were mounted between three of the waveguide coupling cavities in the anode block vacuum housing and the output waveguides. Each of the six 3 cm deep coupling cavities is separated from its corresponding anode cavity by a 1.75 cm wide aperture. In the baseline case, vacuum side window breakdown was observed to initiate at single waveguide output powers close to 20 MW. In the new window configuration, three Air Force Research Laboratory-designed, vacuum-rated directional coupler waveguide segments were mounted between the coupling cavities and the microwave windows. The inclusion of the vacuum side power couplers moved the microwave windows an additional 30 cm away from the anode apertures. Additionally, the Lucite microwave windows were replaced with polycarbonate windows and the microwave window mounts were redesigned to better maintain waveguide continuity in the region around the microwave windows. No vacuum side window breakdown was observed in the new window configuration at single waveguide output powers of 120+MW (a factor of 3 increase in measured microwave pulse duration and factor of 3 increase in measured peak power over the baseline case). Simulations were performed to investigate likely causes for the window breakdown in the original configuration. Results from these simulations have shown that in the original configuration, at typical operating voltage and magnetic field ranges, electrons emitted from the anode block microwave apertures strike the windows with a mean kinetic energy of 33 keV with a standard deviation of 14 keV. Calculations performed using electron impact angle and energy data predict a first generation secondary electron yield of 65% of the primary electron population. The effects of the primary aperture electron impacts, combined with multiplication of the secondary populations, were determined to be the likely causes of the poor microwave window performance in the original configuration.
已对UM/L-3（6叶片，L波段）相对论磁控管进行了实验，以测试一种旨在限制真空侧击穿的新型微波窗配置。在基线情况下，丙烯酸微波窗安装在阳极块真空外壳中的三个波导耦合腔与输出波导之间。六个3厘米深的耦合腔中的每一个都通过一个1.75厘米宽的孔径与其相应的阳极腔隔开。在基线情况下，观察到在单个波导输出功率接近20兆瓦时开始出现真空侧窗击穿。在新的窗配置中，三个由空军研究实验室设计的、真空额定的定向耦合器波导段安装在耦合腔和微波窗之间。真空侧功率耦合器的加入使微波窗离阳极孔又远了30厘米。此外，用聚碳酸酯窗取代了有机玻璃微波窗，并重新设计了微波窗支架，以更好地保持微波窗周围区域的波导连续性。在新的窗配置中，当单个波导输出功率为120兆瓦以上时，未观察到真空侧窗击穿（与基线情况相比，测量的微波脉冲持续时间增加了3倍，测量的峰值功率增加了3倍）。进行了模拟以研究原始配置中窗击穿的可能原因。这些模拟结果表明，在原始配置中，在典型的工作电压和磁场范围内，从阳极块微波孔发射的电子以33千电子伏的平均动能撞击窗口，标准偏差为14千电子伏。使用电子撞击角度和能量数据进行的计算预测，第一代二次电子产额为初级电子总数的65%。初级孔径电子撞击的影响，加上二次电子群体的倍增，被确定为原始配置中微波窗性能不佳的可能原因。
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